In a tersely-worded statement Monday, the White House said it had detected "potential" signs of preparation for a chemical attack similar to those seen in the run-up to an April 4 attack the Trump administration blamed on Damascus.

The White House did not specify what evidence, if any, it has to back up its claim, and has yet to produce proof.

But the Pentagon identified the site of the alleged preparations as the Shayrat air base, the same location struck by dozens of U.S. cruise missiles after the Trump administration said it was used to launch the April 4 attack.

The missile strikes were the first time the U.S. purposefully targeted Assad's forces in the conflict's six-year history.

The White House said should Bashar al-Assad's forces carry out another chemical attack "he and his military will pay a heavy price".

In May, Macron also warned the use of chemical weapons is a "red line" that would draw reprisal from France. He added that any use of chemical weapons would be followed by an immediate response-at least by France